1. Field of the Invention
The present invention relates to a radio communication apparatus for use in vehicular and portable telephone systems, a cordless telephone system and an radio LAN system for example and more particularly to a spread spectrum radio communication apparatus which enables a CDMA (Code Division Multiple Access) communication by using a spread spectrum communication method.
2. Description of the Related Art
A spread spectrum communication method which is tolerant to interference and disturbance has been drawing attention lately as one of communication methods for use in a mobile communication system. In the radio communication system using the spread spectrum communication method, digitized audio and image data are transformed into wide band baseband signals by multiplying a spread code at first in a transmission side apparatus for example. A pseudo random noise code (PN code) and a Gold code are used as the spread code. Then, a radio carrier is digitally modulated by the transformed transmission baseband signal and the modulated wave signal is transmitted from an antenna after filtering by a band-pass filter. A BPSK or a QPSK method is used for example as the digital modulating method.
Meanwhile, the receiving side apparatus implements despreading by using the same code with the spread code used in the transmitting side apparatus and then integrates its output per one symbol length of the received data by integration damp filters. It then recovers the received data by determining the output of the integration by a determining section.
By the way, the following two types of methods are conceivable as the method for integrating the symbol of the digital received signal after the despread by the integration damp filters.
One of the methods is to sample and sum divisional position data within the data at the rate of once per one chip of the spread code. For instance, when the symbol rate of the received signal is 19.2 Kbps and the chip rate of the spread code is 1.23 Mbps, 64 data divisional positions are summed across one symbol length.
The other one is to sum received data sampled with a sampling frequency of four to eight times of the chip rate per one chip of the spread code across one symbol length. For instance, when the symbol rate of the received signal is 19.2 Kbps and the chip rate of the spread code is 1.23 Mbps similarly to those described above and when the received data sampling rate is eight times of the chip rate, i.e., 9.84 Mbps, the sample data of 64.times.8=512 of the received signal are summed across one symbol length.
While there are two methods of the method of integrating multi-samples/one chip and the method of integrating one sample/one chip as the method of the integration damp filter as described above, the method of integrating multi-samples/one chip outperforms the method of integrating one sample/one chip.
That is, the receiving quality is liable to drop by being influenced by a tone interference wave when a cutoff characteristic of a receiving filter provided in a radio section is not good in the method of integrating one sample/one chip. For instance, when a tone interference wave of 900 KHz is added to a radio frequency signal having a band width of 1.25 MHz as shown in FIG. 11 and when no receiving filter for removing the tone interference wave is provided in the radio section, the relationship of receiving S/N with respect to the tone frequency turns out as shown in FIG. 12. As it is apparent from this characteristic, the receiving S/N of one using the integration damp filter of the method of integrating eight samples/one chip is higher by about 9 dB as compared to one using the filter of the method of integrating one sample/one chip.
It is noted that the examples shown in FIGS. 11 and 12 are the results when there is no receiving filter for removing tone interference waves in the radio section and when a sharp filter for removing the tone is put in the radio section, the difference of the performances between the method of integrating eight samples/one chip and the method of integrating one sample/one chip becomes small.
While the method of integrating multi-samples/one chip outperforms the method of integrating one sample/one chip as described above, it consumes electric power of several times (about eight times in case of the method of integrating eight samples/one chip) of that of the method of integrating one sample/one chip when those two methods are compared from the aspect of power consumption because the method of integrating multi-samples/one chip needs to process by using a clock which is faster than a clock used in the method of integrating one sample/one chip by several times (eight times in case of the method of integrating eight samples/one chip).
On the other hand, when the method of integrating eight samples/one chip is used, there has been a possibility that noise components are mixed into the integrated data obtained by the integration damp filter due to a shift of the clock in tracking the clock.
The noise generated due to the shift of the clock will be explained by using FIG. 13.
FIG. 13a shows a transmitted signal and its received signal turns out as shown in FIG. 13b due to the filtering. FIG. 13c shows a PN code to be multiplied with the received signal. It turns out as such with respect to the received signal when there is no shift of the clock. However, actually there is a case that the shift occurs between the received data and the received PN code as shown in FIGS. 13d and 13e due to the shift of the clock in tracking the clock. In the worst case, a PN code "-1" is multiplied with received data of "1" or a PN code "1" is multiplied with received data of "-1" during the period of the shift, thus causing noise components at the both ends of one-chip period.
Then, positions in the vicinity of the both ends of one-chip period are also sampled in case of the method of integrating eight samples/one chip, so that the positions of the noise components generated as described above may be sampled. In such a case, the noise components end up being mixed into the result of the integration.
While there are the two methods of the method of integrating multi-samples/one chip and the method of integrating one sample/one chip as the methods of the integration damp filter as described above, an LSI for a spectrum device including an integration damp filter processing section is made in accordance to either one of the methods in advance.
Therefore, it is difficult to realize a portable apparatus which consumes less power when the LSI in which the method of integrating multi-samples/one chip has been applied to improve the communication quality is used because power consumption in the LSI is large.
It is also difficult to realize a portable apparatus whose communication quality is high when the LSI in which the method of integrating one sample/one chip has been applied to reduce the power consumption because it is weak to the tone interference.
The method of integrating multi-samples/one chip also has had the problem that it cannot exhibit its better performance because the noise components caused by the shift between the received signal and the spread code mix into the result of the integration.